I grabbed the current 12.21.3 beta to see the changes in the metadata pane showing m4b info (read only), but it shows me only what it did before, which isn't much. Like before, if you change the extension to m4a, it shows the full info.
Do I have to do something to make it work? The audiotags plugin is at 12.21.0.0 dll if that counts for anything?
I realise that Jon, I guess I assumed (never assume) that when m4b was implemented, this would be a sort of special case, ghosted or locked somehow or whatever. No biggie. I might experiment with renaming them to m4a and seeing if stuff breaks.